Disordered movement is among the most common and difficult problems faced by the field of rehabilitation medicine. Normal movement depends upon an interaction of neural circuits, primarily in the spinal cord, and the musculoskeletal system. Thus, improved understanding of the relationship between these spinal circuits and the musculoskeletal system is an essential feature for the development of any improved means of treating disordered movement. The overall mission of this Program Project is to investigate the relationship between spinal circuits and the musculoskeletal system. We will study this relationship through investigations of five different domains: output elements of spinal circuits, motor unit activation, proprioceptive feedback, inter-and intramuscular synergies, and reflex plasticity. Each of the domains is represented at some level in each of the individual research projects, but the central focus of each project will be to investigate one of the domains in detail. They each stress the hypothesis that since spinal circuits function in the elaboration of movement fundamentally by their interaction with the musculoskeletal system, it is more appropriate to study that interaction than to study either spinal circuitry or musculoskeletal biomechanics in isolation. Each project emphasizes the new knowledge of such interactions that can be exploited clinically. The results of these studies should mate important new contributions to the science base underlying the practice of rehabilitation medicine.